we're able to say whether these were small bits of metal placed there intentionally or part of the environment. >> reporter: the shrapnel is just one of the clues investigators will be looking at. >> there are fragments in the victims and in the sidewalk and what they will be able to do is probably reconstruct the timer device, what was used and look at the components of the bomb. it will tell them if this is somebody who put this together over the internet or is this a bombmaker? >> reporter: hundreds of state and federal investigators have descended on boston to figure out who could have built the bombs and set them off. >> federal and state and local law enforcement are coordinating closely. the fbi has taken charge of the investigation. >> reporter: this type of attack has long been lawful's worst fear. a soft target where suspects can easily blend in and inflict mass casualties. >> unfortunately these kinds of settings, whether it's a marathon, football, baseball, social event, concert, are attractive targets. we can't get around it. >> reporter: the boston marathon is a heavily policed e